1. Field of the Invention
The subject of the present invention is a joint device for power cables or a power cable termination, comprising at least one tubular rigid support element on which is initially mounted a cold, shrinkable pre-expanded sleeve, the tubular support element being automatically expelled by acting on a releasable immobilizing means. The support thus defined is therefore called an xe2x80x9cautomatic actuated extractionxe2x80x9d support and it is known that, in order to install a cold-shrinkable expanded protective sleeve on joint between two power cables or on the termination of a cable, it is necessary for the diameter of this sleeve to be sufficient to install it correctly on the cable or cables, the sleeve being capable of shrinking so as to grip the joint to be protected.
2. Description of the Related Art
For this purpose, a rigid tube is normally used which serves as a support for the pre-expanded sleeve and which makes it easier to install it, the sleeve subsequently having to be extracted in order to allow the sleeve to contract, which sleeve is made of a silicone-based synthetic rubber.
This extraction operation is not easy to perform because of the radial compressive stresses exerted by the sleeve.
In some constructions, a support for the pre-expanded sleeve, prescored along a helical path, is used, a pull cord allowing the material of the support to unwind along the helical cut. The support can then escape from inside the sleeve when the end of the pull cord is pulled. However, these pull-cord supports are expensive and it takes a relatively long time to extract them.
French Patent Application 2 724 444 (ALCATEL) describes a joint device for power cables, comprising cylindrical tubular rigid elements provided with a sliding film which enables them to be extracted when an axial force is exerted on them. A flexible and strong film having a low coefficient of friction is used as the sliding film, such as a silicone-based film. As the tubular element is progressively released, the pre-expanded sleeve shrinks on to the joint, the sliding film turning inside out.
U.S. Pat. No. 5,467,515 (LUZZI) discloses the use of a cold-shrinkable joint device in which a rigid tapered mandrel can be expelled by the pressure exerted on the tapered surface by the pre-expanded sleeve which grips it. In order to facilitate the expulsion, this document recommends that the external surface of the tapered rigid element be lubricated. In order to keep the tapered support temporarily in position with respect to the shrinkable sleeve, an exterior locking device is provided.
In practice, these various solutions are not entirely satisfactory. Thus, the sliding films recommended in the aforementioned document are complicated to produce and relatively fragile. In addition, it is difficult to prevent any displacement of the sleeve when removing the tubular support element or elements.
Moreover, in order to ensure that the joint is effectively protected by the shrunk sleeve, the pressure exerted by the latter on the entire joint must be high. This results in high compressive forces on the support elements of the shrinkable sleeve. The tapered shape of the support elements with a lubricated surface may be such that the taper causes the support to be expelled effectively by the thrust of the retractable sleeve. However, this tapered shape is not desirable as it also causes possibly excessive deformation of the retractable sleeve at the point of maximum diameter of the tapered support. At this point, the sleeves come back with more difficulties due to the fact that the elastic memory or the material is less when deformation was greater. It is precisely near the ends of the sleeve that tightness of the sleeve is achieved. The shrinking effect must therefore be particularly important in those areas.
What is more, the pressure exerted by the shrinkable sleeve is sometimes such that the internal surface of the latter comes, in places, in direct contact with the external surface of the support element despite the intermediate lubricant, thereby no longer allowing self-expulsion of the support element.
The subject of the invention is a device for lessening or eliminating these drawbacks and for allowing self-contained expulsion actuated by the support element of the cold-shrinkable pre-expanded sleeve which limits the deformation of the sleeve and which operates perfectly despite the very high pressure exerted by the shrinkable sleeve on its support.
The subject of the invention is also a joint which, after the support has been expelled from the shrinkable sleeve, has excellent electrical insulation properties.
Finally, the subject of the invention is a joint device whose operating characteristics remain constant, even when it is stored for several years.
The subject of the invention is also a joint device in which the pressure exerted by the shrunk sleeve after the joint device has been fitted is greater than 1 bar (105 Pa) preferably greater than 4 bar (4xc3x97105 Pa) and may go up to 15 bar (15.105 Pa) so as to ensure excellent protection and to be certain of removing any air pocket which could be formed while the sleeve is shrinking.
The power cable joint device according to the invention comprises at least one tubular rigid support element, lubricated over most of its exterior surface, a cold-shrinkable pre-expanded sleeve, gripping the said support element over most of its length and on its internal end, and a releasable immobilizing means which keeps the said support element immobilized with respect to the pre-expanded sleeve. According to the invention, the said support element is covered over most of its exterior surface with a grease which does not flow under the action of the pressure exerted by the shrinkable sleeve, the said grease being based on a silicone or on a polyalkylene glycol advantageously additioned with a filler comprising for example talc in proportions from 15 to 35% by weight and containing preferably at least one pressure-withstanding additive such as boron nitride in proportions from 2 to 10% by weight.
The grease used is chosen so as to have a pressure-withstanding capability sufficient to withstand the high forces exerted by the shrinkable sleeve during the period of storage of the device, i.e. at least six months and up to about three years. The grease must therefore not flow appreciably under the high pressure exerted by the sleeve. The grease used must furthermore form a continuous lubricating film over the entire surface of the support element despite the pressure exerted by the sleeve, which pressure may be about 5 to 6 bar (5xc3x97105 to 6xc3x97105 Pa) and as much as 15 bar (15xc3x97105 Pa).
In order to achieve these a priori contradictory technical effects, the grease used is a paste having a viscosity of between 100 and 400,000 centistokes, based on a silicone such as polydimethylsiloxane or on a polyalkylene glycol, such as polyethylene glycol, preferably reinforced with a filler consisting of, for example talc eventually additioned with silica in proportions ranging from 15 to 45% by weight and which may further comprise a high-pressure-withstanding additive consisting for example of boron nitride in proportions ranging from 2 to 10% by weight.
The choice of the grease base is linked to the nature of the material used for the expanded sleeve. With a silicone sleeve, it is preferable to use a polyalkylenoglycol based grease. If the sleeve is made of another elastomeric material, such as an ethylene-propylenediene terpolymer, it is on the contrary advisable to use a silicone based grease. The viscosity of the grease is chosen to be more important when the pressure exerted by the sleeve is more important.
The filler used, for example talc, permits to increase the consistency of the grease so that a film of lubricant remains between the sleeve and the support element. With too much filler, the grease becomes sticky. When only said filler is used, its amount may range between 15 and 35% by weight.
The pressure-withstanding additive permits to achieve a suitable sliding effect. If any said additive is used, an amount greater than 15% must be used and preferably between approximately 20 and 40% by weight.
Finally, the grease composition which is used is as follows:
Silicone or polyalkyleneglycol base (Viscosity between 100 and 400,000 centistokes) . . . 60-80% by weight
Pyrogenated silica (thickener) . . . 0-10% by weight
Talc . . . 15-35% by weight
Boron nitride (high pressure withstanding additive) 0-10% by weight
Such a grease has excellent hydrophobicity and good electrical insulation properties. It also has an excellent temperature-withstanding capability and high chemical inertness.
The releasable immobilizing means is preferably such that it creates a localized friction zone between the support element and the shrinkable sleeve and, when it is released, causes localized shrinkage of the sleeve near the internal end of the support element, which is then thrust axially outwards. The grease defined above ensures sufficient lubrication for this thrust to cause effective self-contained expulsion of the support element, whatever the shape of the support element.
The support element generally comprises, for a joint between two cables, two tubular rigid half-supports arranged inside the pre-expanded sleeve with their internal ends facing each other.
The dimensions of the two half-supports may be the same or different.
The half-supports may be cylindrical, with a circular or elliptical cross-section. They may be tapered, at least partly, with a circular or elliptical cross-section, the taper being oriented in such a way that the diameter of the half-support is smaller towards the interior of the joint.
The releasable immobilizing means preferably comprises a means for temporarily fastening the two half-supports together, which may be produced in various ways.
In a first embodiment, the means for temporarily fastening the two half-supports together comprises an internal strap, the ends of which emerge on the outside via holes provided in the half-supports,
In a second embodiment, the means for temporarily fastening the two half-supports together comprises at least one tab which projects from the internal edge of a half-support, is inclined with respect to the axis of the half-support and engages in a recess of corresponding shape cut into the internal edge of the other half-support, so that a relative rotation of the half-supports with respect to each other causes them to separate.
In another embodiment, the means for temporarily fastening the two half-supports together is formed by a frangible fastening of the internal edges of the two half-supports.
It is also possible to provide, on at least one of the half-supports, near its internal edge, a portion which can be torn by pulling a tie extending as far as the outside, the exterior surface of this tearable portion being devoid of grease. This non-lubricated portion is sufficient to immobilize the relevant half-support despite the thrust exerted by the shrinkable sleeve.
The same effect is obtained when each half-support comprises an annular friction element in contact with the pre-expanded sleeve, it being possible for the said element to be expelled by pulling on a tie extending as far as the outside.
It is important for the quality of protection of the joint that the shrunk sleeve be perfectly centred with respect to the joint. However, it is realized that any displacement of the half-support or supports during it or their expulsion runs the risk of modifying any centring performed before shrinkage. It is therefore useful to provide, in addition, means for precisely centring the sleeve with respect to the joint.
For this purpose, the joint device according to the invention comprises an additional means for immobilizing one of the half-supports with respect to the pre-expanded sleeve, it being possible for the effect of this additional sleeve to be obviated after expelling the other half-support and checking that the joint is centred.
In one embodiment, at least one longitudinal external strap connecting the external edges of the half-supports together and a removable clamping means fixing the said external straps to the pre-expanded sleeve and defining portions of unequal length on the straps are provided.
In another embodiment, at least one longitudinal external strap connects the external edges of the half-supports together and at least one return strap is fixed on one side to the external edge of a half-support and on the other side to the exterior surface of the pre-expanded sleeve, near the centre of the joint device.
In another embodiment, at least one longitudinal external strap connecting the external edges of the half-supports together and a removable localized-friction means placed on each half-support and in direct contact with the shrinkable sleeve are provided.
In all cases, the support element preferably includes means for separating it from the power cable after it has been expelled from the shrinkable sleeve.